The objective of this research is to study the viral and particularly the cellular control of the transformed phenotype in cells transformed by the oncogenic viruses Polyoma and SV40. We also want to study the regulation of the expression of viral functions, including viral DNA replication, in the transformed cells. Our work is planned along three main lines of research: 1) The study of the viral and cellular factors controlling the stable association between the viral and the cellular genome in rat cells transformed by Polyoma virus: In this system, "free" and integrated viral DNA molecules coexist, and the free molecules probably result from excision and limited replication of the integrated viral DNA. 2) The study of the regulation of viral transcription and T-antigen expression in SV40 transformed cells. Expression of viral functions appears in fact to be controlled by the position of the transformed cells in the mitotic cycle. 3) The study of the mechanisms by which Polyoma virus makes transformed cells unable to reach a state of viable G1 arrest. This inability is one of the most distinctive properties of Polyoma or SV40 transformed cells, and we want to understand which viral gene-product(s) is involved in this phenomenon, and which type of G1 blocks can be partially or totally overcome by the virus.